By using this plugin you'll be able to allow only a specific number of visitors to navigate through your website.

queuefair

Queue-Fair FREE Virtual Waiting Room

Stop Your Website Crashing - for Free! Manage traffic peaks gracefully. Treat your visitors with fairness with the top-rated Virtual Waiting Room.

alxuta

By using this plugin you’ll be able to allow only a specific number of visitors to navigate through your website. 
The extra influx of visitors will be kept in a virtual queue until a new space will be available.
You could always use the following shortcode [virtual-queue-position], in order to display the current user position into the queue.
Installation
<ol>
<li>Upload the <code>virtual-queue</code> directory to the <code>/wp-content/plugins/</code> directory</li>
<li>Activate the plugin through the ‘Plugins’ menu in WordPress</li>
<li>Update the plugin settings</li>
<li>Set a Cron Job that will access the link https://domain.com/virtual-queue/maintenance/ where domain.com is your own domain name.</li>
<li>Go to Settings -> Permalinks / Permalink Settings and click Save Changes</li>
</ol>

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